(1) Field of the Invention
This invention relates to a supercooling degree control type expansion valve, and more particularly to a supercooling degree control type expansion valve for use in a refrigeration cycle of an air conditioning system for an automotive vehicle.
(2) Description of the Related Art
As the refrigeration cycle of an air conditioning system for an automotive vehicle, there has been widely employed one using a receiver/dryer arranged at an outlet side of a condenser for storing a superfluous refrigerant and subjecting the stored refrigerant to air-liquid separation, a thermal expansion valve for controlling the flow rate of the refrigerant flowing into the evaporator according to the pressure and temperature of a low-pressure refrigerant delivered from the evaporator.
On the other hand, another refrigeration cycle is also known which uses an accumulator arranged at an outlet side of an evaporator, for storing a superfluous refrigerant and subjecting the stored refrigerant to air-liquid separation, and a supercooling degree control type expansion valve comprised of a restriction passage (orifice) for control of the flow rate of the refrigerant according to the degree of supercooling and dryness of a high-pressure refrigerant delivered from a condenser, and a differential pressure regulating valve for carrying out control such that the refrigerant is cooled to a predetermined supercooling degree.
FIG. 10 is a cross-sectional view showing the construction of a conventional supercooling degree control type expansion valve.
The conventional supercooling degree control type expansion valve 1 has a body 2 in the form of a hollow cylinder having a portion at a left side, as viewed in FIG. 10, which is connected to the upstream side of a refrigeration cycle, with part of a side wall thereof being formed with a large opening into which a strainer 3 is fitted. The body 2 has a refrigerant passage extending through a central portion of the body 2 and having an intermediate portion formed with a stepped portion which constitutes a valve seat 4. A valve element 5 is axially movably arranged in the passage in a manner opposed to the valve seat 4 from the downstream side, and the valve element 5 is urged in a valve-closing direction by a spring 6 arranged on a downstream side thereof. Further, the body 2 has a lower end thereof fitted with a spring-receiving member 7, and the spring-receiving member 7 has an annular orifice 8 formed therethrough which communicates with the outside. The body 2 has an O ring 9 fitted on the periphery thereof.
In the supercooling degree control type expansion valve 1 constructed as above, when the refrigeration cycle is operating at a low load condition or the compressor is rotating at a low rotational speed, the refrigeration cycle is at a low pressure condition as a whole, so that the valve element 5 is urged by the spring 6 against the valve seat 4 to hold the valve 1 in a closed state, which inhibits the refrigerant from flowing therethrough.
When the refrigeration cycle is operating at a normal load condition, a high-pressure refrigerant from a condenser, not shown, is first filtered by the strainer 3, and then introduced into the upstream side of the valve element 5. When the pressure of the refrigerant introduced into the upstream side of the valve element 5 becomes higher or stronger than the urging force of the spring 6, the valve element 5 leaves the valve seat 4 whereby the refrigerant flows to the downstream side of the valve seat 4, and further passes through the annular orifice 8 in the valve-receiving member 7, where the refrigerant undergoes thermal expansion and then flows to an evaporator, not shown. During the process, the valve element 5 controls the flow rate of the refrigerant depending on the balance between the differential pressure between the upstream side and the downstream side of the valve seat 4, and the urging force of the spring 6.
When the temperature of the outside air is low e.g. during winter, or when the rotational speed of the engine is low e.g. during idling operation of the engine, the pressure of the whole refrigeration cycle is low. Therefore, when the introduced pressure is low e.g. at a low load condition, there can arise a situation in which the valve element does not open but remains closed to inhibit the flow of the refrigerant.
The refrigerant for circulation, however, contains oil for lubrication of the compressor, and hence in the case of the conventional supercooling degree control type expansion valve, if the refrigerant ceases to flow, the amount of oil returning to the compressor decreases, which in worst cases, causes seizure of the compressor due to shortage of the oil.
Further, when the vehicle is running at a high speed, the rotational speed of the compressor is increased to increase the pressure within the refrigeration cycle. Therefore, it is necessary to configure the supercooling degree control type expansion valve such that it withstands high pressure from the viewpoint of safety. Further, the power of the compressor is increased to a larger degree than required for cooling, which degrades the coefficient of performance of the refrigeration cycle as well as fuel economy.
The present invention has been made in view of the above problems and an object thereof is to provide a supercooling degree control type expansion valve which is capable of preventing seizure of a compressor, at a low load condition.
Further, another object of the present invention is to provide a supercooling degree control type expansion valve which is capable of preventing the pressure from rising when the vehicle is running at a high speed.
To solve the above problem, the present invention provides a supercooling degree control type expansion valve including a restriction passage arranged in a refrigerant passage through which a refrigerant flows, for subjecting the refrigerant introduced to adiabatic expansion, and a differential pressure regulating valve arranged on an upstream side of the restriction passage, for carrying out control such that the refrigerant introduced has a predetermined cooling degree, characterized by comprising differential pressure regulating valve bypass means for allowing the refrigerant to flow therethrough at a minimum refrigerant flow rate required for a compressor even when the differential valve is closed.
According to this supercooling degree control type expansion valve, although the differential pressure is closed when the rotational speed of the engine is low and the compressor is at a low load condition, it is still possible in such a case to cause part of the introduced refrigerant to flow via the differential pressure regulating valve bypass means, which makes it possible to return the oil contained in the refrigerant to the compressor, to thereby prevent seizure of the compressor.
Further, according to the invention, the restriction passage includes passage area-varying means for increasing a passage area thereof in response to received pressure which is higher than a predetermined pressure. Owing to the provision of the passage area-varying means, when the refrigerant at a high pressure is introduced due to a high rotational speed of the compressor which is caused e.g. when the vehicle is running at a high speed, the passage area-varying means increases the passage area of the restriction passage to thereby increase the flow rate of a refrigerant flowing through the restriction passage, which makes it possible to prevent the pressure from rising, and hence prevent breakage due to pressure, and degradation of the coefficient of performance and fuel economy.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in condjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.